The invention relates to synchronization of a receiver with a signal to be received.
In the CMDA (Code Division Multiple Access) the user""s narrowband data signal is modulated onto a relatively wide band with a spreading code which has a wider band than the data signal. In prior art CDMA systems bandwidths of over 1 MHz are used. In the WCDMA (Wide-band CDMA) the bandwidth is considerably greater so that the users of mobile communications networks can be offered a wider range of services.
The spreading code used in the CDMA system usually consists of a long pseudorandom bit sequence. The bit rate of the spreading code is much higher than that of the data signal. The spreading code bits are called xe2x80x98chipsxe2x80x99 to distinguish them from the data bits and symbols. Each of the user""s data symbols is multiplied by spreading code chips. In that case a narrowband data signal spreads over the frequency band used by the spreading code. Each user has a spreading code of their own. The length of a spreading code may be one or more data bits. Several users transmit simultaneously on the same frequency band, and the data signals are distinguished from one another in receivers on the basis of a pseudorandom spreading code.
In spread spectrum systems, such as the CDMA system, the receiver has to synchronize with the signal to be received so that the signal can be modulated and detected. Code synchronization is usually divided into two parts: code acquisition and code tracking. In code acquisition the object is to achieve a maximum timing difference of one spreading code chip between the spreading code of the receiver""s code generator and the spreading code of the received signal. To achieve this, the spreading code of the receiver""s code generator is delayed chip by chip, and the delay between the spreading code of the signal and the spreading code of the code generator is searched for. In code tracking the object is to achieve as small timing inaccuracy as possible, which is only a fraction of a chip.
Code acquisition is performed using e.g. a simple correlator. When the spreading code of the signal to be received and the spreading code of the receiver""s code generator are not synchronized, the output of the correlator receives a low value. When the spreading code of the signal to be received and the spreading code of the receiver""s code generator are synchronous, i.e. the delay of code generator""s spreading code is correct, the initial value of the correlator is high. A corresponding result is obtained when a matched filter is used.
In such code acquisition problems are caused by interferences and noise. Multiple access interference MAI, for example, causes high momentary values at the output of the correlator, even though the spreading code of the signal to be received and the code of the code generator would be non-synchronous. This leads to a false alarm and protracted code acquisition. To reduce the influence of false alarms, the delay obtained in code acquisition has to be confirmed with another correlation measurement, which prolongs synchronization.
An object of the invention is to provide a method and a receiver implementing the method to reduce the problems related to code acquisition and to speed up code acquisition. This is achieved with a method according to claims 1 and 2 and with a receiver according to claims 13 and 14.
The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on performing code acquisition two-dimensionally both in the time domain and in the angular domain. In the angular domain code acquisition is performed using several adjacent antenna beams, i.e. angular cells.
The method and arrangement of the invention provide several advantages. Code acquisition can be sped up considerably in noisy multi-use environment even when the number of angular cells is not optimal with respect to the signal to noise ratio.